The invention relates to projection television systems including three separate cathode ray tubes (CRTs), and more particularly, to circuitry for combining automatic kinescope bias (AKB) circuitry with circuitry for sensing and limiting average individual beam current sensing.
Many CRT-based video display systems employ an automatic kinescope bias (AKB) control systems for maintaining proper black image current levels for each electron gun of an associated image displaying kinescope or CRT. The purpose of such AKB control systems in a CRT is to prevent the displayed picture colors and picture grey scale information from being adversely affected by variations in the kinescope bias due to such factors as aging, temperature changes, etc. Conventional AKB control systems include a beam sampling element coupled to a control amplifier in each AKB feedback loop for automatically adjusting the kinescope black level of the electron gun. Adjustment is implemented typically during the blanking period to prevent variability of the black level sampling pulses.
A conventional AKB control system used in a video display system is disclosed in U.S. Pat. No. 4,633,321 (the '321 patent). The '321 patent discloses an apparatus for automatically controlling the bias of an image display device such as a kinescope in a television receiver. The '321 patent apparatus comprises a grid system responsive to a grid drive signal in order to prevent visible artifacts during non-blanking AKB operating periods. More particularly, the kinescope electron gun is caused to conduct a white-going current during plural horizontal line intervals in response to the grid drive signal applied to the kinescope electron gun during a given portion of the AKB bias monitoring interval. The drive signal is blanked during horizontal retrace intervals to eliminate the white current at such times, thereby eliminating visible horizontal line retrace artifacts which would otherwise appear to the uncorrected retrace pattern of planar kinescope.
U.S. Pat. No. 5,488,417 (“the '417 patent”) discloses an AKB system which avoids the need for grid drive circuitry to prevent visible artifacts. The '417 patent uses a controllable current source in an AKB circuit to apply a measurement current to a kinescope driver amplifier during selected lines of the vertical interval of a video input signal. The amplified current signal induces a beam current in a kinescope coupled to the driver amplifier. A comparison circuit compares samples of the beam current obtained during the selected lines with a reference signal and applies a correction current to the driver amplifier for regulating a parameter, e.g., black level, of displayed images based on the comparison.
A signal source, coupled to the controllable current source, inhibits production of the measurement current during retrace portions of the selected lines and enables production of the measurement current during trace portions of the selected lines. The benefit of such a construction is that use of the signal source provides for suppression of visible artifacts due to AKB operation.
U.S. Pat. No. 6,188,435 B1 discloses a circuit for controlling beam current using current “pictures” for the R, G, B beam currents in the kinescope or color picture tube. The means for accomplishing the beam current control monitors each individual R, G and B beam current individually to maintain its corresponding picture sharpness and/or peak white maximum beam current, average beam current values per line and per picture. The current pictures correspond to the real currents in the color picture tube, the sum of which is compared with beam current information obtained from the associated high voltage transformer to both correct fluctuations in the millisecond range and long-term drifts. Use of the current pictures enables the beam current control to quickly regulate picture sharpness for the three R, G, B colors as well as total beam current in the picture tube.
In order to generate the current pictures, the beam current control circuit taps or clamps the R, G, B signals in an amplifier including a controllable gain, amplified and weighted by means for gamma correction. The real current ratios in the picture tube are simulated in the current control circuit by the gamma correction means. A weighted sum is formed of the signals and serves as a comparison value with respect to a beam current information value, Icrt. The two information signals are compared, and an output signal is generated by the comparison. The output signal is used to control the gain of the amplifier. The current control circuit thereby monitors the values of each individual R, G, B signals to regulate the R, G, B beam currents and picture sharpness, and the weighted sum together with the output signal of the control circuit are monitored by a decision circuit.
In projection television display systems having three separate CRTs for producing R, G, B light, certain signals such as a high level flat field in an individual primary color can result in the respective CRT providing much more than its nominal share of light. While it is common practice to sense and control the total beam current, under such flat field conditions, one CRT may draw most if not all the beam current ordinarily allocated for all three CRTs. When this occurs, overheating and fracturing of the CRT may occur. This is particularly troublesome in the case of video accessory devices which provide a blue flat field when no program content is being provided.
To accommodate these undesirable signal conditions, it would be effective to determine the relative share of current provided to each CRT. In the prior art, for example, in a Philips/Magnavox GR-9D, the cathode current of the blue CRT is directly measured, with the preceding R, G, B video gains controlled to limit the average blue current to a safe level. The skilled artisan, however, will realize that it is also desirable to utilize individually sampled cathode currents for automatic CRT cut-off stabilization, that is, automatic kinescope bias (AKD) control.